Did you add this cron job yourself, or did you just use the one that was installed by sysstat (usually in /etc/cron.d/sysstat)? Maybe there's another job running that's recording data every 10 minutes.

You shouldn't see 3 entries every x minutes anyway... the 3 just means that it should write 3 entries 20 minutes apart. i.e. first cron job of the day runs at 08:00, collects data for 1200 seconds, writes an entry, collects data for 1200 seconds, writes second entry, collects data for 1200 seconds, writes third entry and exits... at which time it would be 09:00 when the next cron job kicks in and repeats the exercise.

So the only mystery here is why it's recording at 10 minute intervals... unless sar insists on reporting in 10 minute intervals by default, but I didn't think it could do that unless the data was actually recorded with that frequency.